The invention concerns a method of producing sub-micron photolithographic patterns by means of a wet-developable single-layer resist system or by means of a two-layer resist system.
Chemically amplified resists are widely used in microelectronics (cf. "Solid State Technology," Vol. 34 (1991), No. 8, pages 53 to 60). Chemical amplification is used both in wet-developable single-layer resists and in entirely or partly dry-developable resists. The resists can, in this context, operate on the principle of acid-catalyzed cleavage, in which chemical groups, for example carboxyl groups or phenolic hydroxyl groups, that are polar but are blocked with a protective group are deblocked by means of a photolytically produced acid, and the resist changes polarity in the exposed areas. This change in polarity can be used, for example, for development or for a selective silylation.
For rapid detachment of the protective group, the resist is subjected after exposure to a temperature treatment (post-exposure bake=PEB) which initiates or accelerates the deblocking. Examples of blocking groups are tert-butyl ester and tert-butoxycarbonyloxy (t-BOC) groups. The polarity change results in differing solubility in the exposed and unexposed areas, i.e. the resist can, with suitable developers, be specifically developed either positively (exposed areas dissolved away) or negatively (unexposed areas dissolved away).
The resist generally consists of at least two solid constituents, i.e. of a base polymer that has the tert-butyl ester or t-BOC groups, and a photoactive acid former. Resists containing polymers with such groups are known, for example, from U.S. Pat. No. 4,491,628. The acid former is preferably anonium compound such as diphenyliodonium and triphenylsulfonium trifluoromethanesulfonate, or the corresponding hexafluoroarsenates. Resists of this kind are particularly suitable for photostructuring in the sub-micron and sub-half-micron range.
Published EP Application 0 494 383 discloses a photolithographic pattern generation using the two-layer resist technique in which--following exposure, a subsequent temperature treatment (post-exposure bake=PEB), and development--silylation occurs from the liquid phase followed by anisotropic etching in an oxygen plasma. Positive or negative structures are produced depending on the type of silylation solution. The resist generally consists of at least two solid constituents, i.e. of a base polymer and a photoactive acid former. The base polymer contains carboxylic acid anhydride and tert-butyl ester partial structures, and the acid former is preferably anonium compound such as diphenyliodonium and triphenylsulfonium trifluoromethanesulfonate. A resist of this kind is particularly suitable for photo patterning in the sub-micron and sub-half-micron range with very steep sidewalls.
In the production of patterns features in the manner mentioned above, as in other known resist systems operating according to the principle of acid-catalyzed cleavage or that of chemical amplification, the so-called "delay time effect" has been noted. Specifically, if the delay time between exposure and temperature treatment (PEB) exceeds a certain value, considerable discrepancies occur between the nominal pattern dimension (pattern size on the mask) and the imaged pattern (pattern size in the resist after development). The longer this delay time, the greater the discrepancy. Above a certain value for the delay time, for example about 30 minutes in the case of anhydride group-containing resists of the type mentioned above, almost no patterns are recognizable after development. The tolerable delay time for these resists is approximately 5 to 10 minutes; for production-engineering reasons, however, a delay time of this kind cannot be accepted.
For a positive resist effective in the deep UV range, for example, a delay time of 1 hour is indicated (see "Microelectronic Engineering," Vol. 21 (1993), pages 267 to 270). Substantially longer delay times would be desirable for production, however; it would be advantageous if exposed resists could, if necessary, even be stored overnight, i.e. if the delay time were 15 hours or more.
This problem is generally known, and is attributed to alkaline contamination in the air which deactivates the photochemically generated strong acid during the delay time. It has therefore already been proposed to solve this problem by filtering the air using activated carbon (see "Proc. SPIE," Vol. 1466 (1991), pages 2 to 12). This, however, requires large investments. Other actions, for example the addition of additives, also have not resulted in a decisive attenuation of the delay time effect (see "Proc. SPIE," Vol. 1466 (1991), pages 13 to 25). It is possible to extend the delay time by applying an additional layer, but only to a minor extent. Moreover this action constitutes an additional process step, which is however undesirable in production because it leads to yield losses.